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From: "mhillen at linux dot ibm.com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/92294] alias attribute generates incorrect code
Date: Thu, 19 Nov 2020 13:02:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D92294

--- Comment #8 from Marius Hillenbrand <mhillen at linux dot ibm.com> ---
>>From my current understanding, gcc addresses a and b in two different ways,
which is not handled correctly by the dependency analysis / alias analysis
while employed by the cse1 pass, and then causes cse1 to incorrectly elimin=
ate
the read from b (reusing the const 1 from b=3D1).

the rtl addresses a via the section anchor yet b via its symbol reference:

b =3D 1 becomes
(set (reg/f:DI 62) (symbol_ref:DI ("b") [flags 0x602]  <var_decl .. b>))
(set (mem/c:SI (reg/f:DI 62) [1 b+0 S4 A32]) (const_int 1 [0x1]))

vs

a=3D2 becomes
(set (reg/f:DI 64) (symbol_ref:DI ("*.LANCHOR0") [flags 0x182]))
(set (mem/c:SI (plus:DI (reg/f:DI 64) (const_int 4 [0x4])) [1 a+0 S4 A32])
(const_int 2 [0x2]))

when visiting the write to a, cse1 should identify the aliasing and thus dr=
op
the "remembered" b. for that, cse iterates its hash table and compares all
present MEM rtx to the new address (cse.c:invalidate()).

that step compares (canonicalized) addresses
(const:DI (plus:DI (symbol_ref:DI ("*.LANCHOR0") [flags 0x182]) (const_int 4
[0x4])))
and (symbol_ref:DI ("b") [flags 0x602]  <var_decl b>)

the comparison of these two (in alias.c:write_dependence_p()) falsely claims
that there is no anti-dependence between the two.

I am not certain yet how that comparison is supposed to work with section
anchors. It looks like we either fail to reconnect (anchor + 4) to the
symbol_ref of "a" (so we could identify both symbol_refs as equivalent) or =
lose
the offset relative to the anchor (so we could see that both have the same
offset relative to the anchor).=